Suppressing output offset in an audio device

ABSTRACT

A digital offset is combined with an audio signal in the digital domain to cancel an output offset caused by one or more analog components processing the same audio signal. In this manner, the offset at the output of the audio signal path (e.g., at a power amplifier output) is reduced or eliminated. Consequently, audible artifacts, such as click-and-pop artifacts, can be reduced or eliminated. In audio devices operating in ground-referenced capless mode, power consumption is reduced because of reduced or eliminated direct current (DC) leakage current through speakers or headsets of such audio devices. In some circumstances, the digital offset in the digital domain may be applied at substantially all times of operation of the audio signal path.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present Application for patent claims priority to ProvisionalApplication No. 60/976,334, entitled “Apparatus and Methods for ReducingOutput Offset” filed Sep. 28, 2007, and assigned to the assignee hereofand hereby expressly incorporated by reference herein.

BACKGROUND

1. Field

The present disclosure generally relates to digital audio devices, andmore particularly, to techniques for suppressing transient audioartifacts and reducing power consumption in audio devices.

2. Background

Digital audio devices are electronic devices that produce sound fromdigital information. They include, but are not limited to, radioreceivers, stereo systems, computers, audio playback devices, such asMPEG-1 Audio Layer 3 (MP3) players; landline telephones, cellularphones, personal digital assistants (PDAs), satellite radio receivers,audio visual systems, such as video players, and the like.

Two performance considerations in digital audio devices are the presenceof audible artifacts and power consumption.

An audible artifact is a perceivable noise introduced into the soundoutput from the audio device, frequently caused by operation of theaudio device itself. Audible artifacts are usually undesirable andrepresent a deviation from the fidelity of the audio input to thedevice.

Click-and-pop is a specific type of audible artifact. Click-and-pop isan unpleasant audible artifact produced by a speaker, which is oftenheard when an audio device powers up or powers down. Click-and-pop isgenerally caused by an abrupt transient voltage, e.g., a direct current(DC) offset across a speaker that may occur when an audio poweramplifier transitions between operational modes, such as a power-offmode and power-on mode. Other operational modes may include a shutdownmode, where power is still applied to the audio power amplifier, but theaudio signal itself is discontinued within the device. Click-and-pop cansometimes be heard when the audio signal is discontinued or resumed,i.e., transitioning to or from the shutdown mode, respectively.

Click-and-pop can be especially annoying when using headsets with audiodevices. It has become quite common for consumers to use telephones,including cellular and convention landline telephones, through headsetsor headphones (“headsets” collectively). Similarly, headsets are usuallyused with audio playback devices, such as MP3 players. Headsets may bewired or wireless, for example, Bluetooth™-enabled headsets. Due to aheadset's close proximity to a user's ears during use, headsetclick-and-pop can be greatly annoying or even harmful to hearing.Therefore, there is a need in the art to reduce or eliminateclick-and-pop in digital audio devices.

Another criterion affecting the design of digital audio devices,particularly portable communication and entertainment devices, is powerconsumption. For example, expected battery life of a portable device isinversely related to the power consumed by the device. Therefore, thereis also a need in the art to reduce the power consumption in voicecommunication and audio reproducing/playing devices.

SUMMARY

Disclosed herein is a new and improved approach to the problems ofclick-and-pop, and in some circumstances, power consumption in digitalaudio devices. The approach significantly reduces click-and-popartifacts caused by operational mode transitions of an audio device, andcan also reduce power consumption by audio amplifiers and speakers.

According to one aspect of the approach, an apparatus includes a digitalprocessing portion configured to add a digital offset to a digitizedaudio signal. The digital offset is based on an output offset introducedinto an analog audio signal by one or more analog components. The analogcomponents are configured to produce the analog audio signal in responseto the digitized audio signal.

According to another aspect of the approach, a method includes adding adigital offset to a digitized audio signal prior to the digitized audiosignal being received by one or more analog components. The digitaloffset is based on an output offset introduced into an audio signal bythe analog components.

According to another aspect of the approach, an apparatus includes meansfor storing a digital offset. The digital offset is based on an outputoffset introduced into an audio signal by one or more analog components.The apparatus also includes means for adding the digital offset to adigitized audio signal prior to the digitized audio signal beingreceived by the analog components.

According to another aspect of the approach, a computer-readable mediumembodying a set of instructions executable by one or more processorsincludes code for storing a digital offset, where the digital offset isbased on an output offset introduced into an audio signal by one or moreanalog components. The computer-readable medium also includes code foradding the digital offset to a digitized audio signal prior to thedigitized audio signal being received by the analog components.

According to another aspect of the approach, a device for suppressingtransient audible artifacts includes digital circuitry configured to adda digital offset to a digitized audio signal. The device also includesanalog circuitry configured to receive the digitized audio signal fromthe digital circuitry and to process the digitized audio signal toproduce an analog audio signal. The digital offset is based on an outputoffset introduced into the analog audio signal by the analog circuitry.

According to another aspect of the approach, an apparatus includes afirst digital processing portion configured to add a first digitaloffset to a first digitized audio signal corresponding to a left stereochannel, and a first analog processing portion configured to receive thefirst digitized audio signal from the first digital processing portionand to process the first digitized audio signal to produce an analogaudio signal. The apparatus also includes a second digital processingportion configured to add a second digital offset to a second digitizedaudio signal corresponding to a right stereo channel, and a secondanalog processing portion configured to receive the second digitizedaudio signal from the second digital processing portion and to processthe second digitized audio signal to produce an analog audio signal. Thefirst digital offset is based on a first output offset introduced intothe first analog audio signal by the first analog processing portion,and the second digital offset is based on a second output offsetintroduced into the second analog audio signal by the second analogprocessing portion.

Other aspects, features, and advantages of the disclosed approach willbe or will become apparent to one with skill in the art upon examinationof the following figures and detailed description. It is intended thatall such additional features, embodiments, processes and advantages beincluded within this description and be protected by the accompanyingclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

It is to be understood that the drawings are solely for purpose ofillustration. Furthermore, the components in the figures are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the apparatus and methods disclosed herein. In thefigures, like reference numerals designate corresponding partsthroughout the different views.

FIG. 1 is a conceptual block diagram illustrating certain components ofan exemplary apparatus for reducing or eliminating output offset in anaudio device.

FIG. 2 a conceptual block diagram illustrating certain components of anexemplary audio device configured to reduce or eliminate output offsetfrom an audio signal path.

FIG. 3 is a process diagram illustrating a method for processing anaudio signal to suppress undesirable audible artifacts caused by anoutput offset.

FIG. 4 a conceptual block diagram illustrating certain components ofanother exemplary audio device configured to reduce or eliminate outputoffset from an audio signal path.

DETAILED DESCRIPTION

The following detailed description, which references to and incorporatesthe drawings, describes and illustrates one or more specificembodiments. These embodiments, offered not to limit but only toexemplify and teach, are shown and described in sufficient detail toenable those skilled in the art to practice what is claimed. Thus, forthe sake of brevity, the description may omit certain information knownto those of skill in the art.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment or variant described hereinas “exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments or variants. All of the embodimentsand variants described in this description are exemplary embodiments andvariants provided to enable persons skilled in the art to make and usethe invention, and not necessarily to limit the scope of legalprotection afforded the appended claims.

For purposes of convenience and clarity only, directional terms, such astop, bottom, up, down, over, above, below, beneath, rear, and front maybe used herein with respect to the accompanying drawings. These andsimilar directional terms should not be construed to limit the scope ofthe invention. The references to left and right channels relate to leftand right sound stereo channels, and references to upper and lower pathsare used to distinguish the paths by their relative locations on thedrawings.

One reason for click-and-pop in some audio devices is the presence of adirect current (DC) offset voltage at the output of an analog domainportion of an audio signal path within the audio device. This outputoffset is typically caused by non-ideal performance of components withinthe analog portion of the audio signal path. Even when no audio signalis present at the input to the analog portion, the output offset may bepresent. Thus, when the audio device is powered up, the output of theaudio path abruptly transitions from zero volts to some voltagedetermined by the output offset. This abrupt transient signal may resultin click-and-pop. Likewise, when the audio device is powered down, theaudio path output transitions from the output offset value to zerovolts, also causing click-and-pop. Other modal changes can also causeclick-and-pop due to the output offset.

Disclosed herein are one or more devices, apparatuses, methods, andarticles of manufacture that insert a compensating digital offset intoan digitized audio signal on an audio signal path in the digital domainto cancel estimated output offset caused by an analog domain of theaudio signal path. In this way, the output offset at the output of theaudio path, e.g., at the power amplifier output, is reduced oreliminated. Click-and-pop is thus reduced or eliminated altogether. Inaudio devices that operate in ground-referenced capless mode, powerconsumption is reduced because suppressing the output offset reduces oreliminates DC leakage current through speakers and/or headsets of suchaudio devices.

The devices, apparatuses, methods, and articles of manufacture describedherein may be embodied in any suitable audio device including, but notlimited to, radio receivers, stereo systems, computers, such as PCs orlaptops, audio playback devices, such as MPEG-1 Audio Layer 3 (MP3)players, telephones, cellular phones, personal digital assistants(PDAs), satellite radio receivers, audio visual systems, such as videoplayers, and the like. Audio processed and produced by the devices,apparatuses, methods, and articles of manufacture described herein mayinclude any sound perceivable to the human ear or otherwise, such asvoice, music, background sounds, noises and the like.

FIG. 1 is a conceptual block diagram illustrating certain components ofan exemplary apparatus 100 for reducing or eliminating output offset inan audio device. The audio device is preferably a digital audio devicethat receives digital samples representing sound, and in turn processesthe digital samples using digital and analog circuitry to generate soundoutput.

The apparatus 100 includes a digital processing portion 110 and ananalog processing portion 130 configured to process audio signals on aaudio signal path traversing through the audio device.

The digital processing portion 110 includes an input 112 configured toreceive a digitally sampled sound stream (e.g., a digitized audiosignal) and digital circuitry to process the digital audio sample streamin the digital domain. The digital samples represent discrete samples ofan analog audio signal. The apparatus 100 is not limited to anyparticular digital audio format. The format of the sample stream dependsupon the audio device in which the apparatus 100 is implemented, andthus, the sample stream may, in some circumstances, be raw audiosamples, such as pulse code modulation (PCM) samples, or in othercircumstances, digitally encoded and/or compressed audio, such MP3audio.

The digital processing portion 110 further includes an output 114 andvarious circuitry for processing the digital audio stream. For example,the sampled audio stream may be truncated one or more times, filteredone or more times, amplified one or more times, and upsampled one ormore times. Filtering may include low pass filtering, high passfiltering, and/or passing the stream through filters characterized byother kinds of filter functions. Amplification in the digital domain mayinclude the use of a programmable gain amplifier (PGA).

As shown in FIG. 1, the digital processing portion 110 also includes asummer 120. The summer 120 is configured to add into the audio samplestream a digital offset. The digital offset is a digital value selectedto cancel an estimated DC output offset caused by analog components inthe analog processing portion 130. The summer 120 has an input 122 forreceiving the digital offset, and an output 124. The digital offset may,in some circumstances, be a value stored in a memory 125 of theapparatus 100, such as in a memory of the digital processing portion 110or in a memory of another block. The digital offset can comprise apredetermined number of bits, where the number of bits is based on gainapplied to the audio signal by the analog processing portion. The summer120 is configured to add into the audio sample stream the digitaloffset. The digital offset may be positive or negative, and it is basedon the output offset. If the sampled audio stream in front of the summer120 corresponds to (i.e., is a digital representation of) an analog,time-domain waveform S(t), and the digital offset at the input 122corresponds to an analog value of OFFSET_(DC), then the offset digitalaudio stream at the output 124 of the summer 120 would correspond to ananalog audio waveform S(t)+OFFSET_(DC).

More than one sampled audio stream may be processed in this way. Forexample, two stereo channels may be processed and offset, with each ofthe two channels receiving a different or the same digital offset value.

The summer 120 may be positioned at the front end of the digitalprocessing portion 110, i.e., near or at the input 112. The summer 120may also be positioned at the rear end of the digital processing portion110, i.e., near or at the output 114. Furthermore, the summer 120 may bepositioned anywhere else between these two extreme points of the digitalprocessing portion 110.

The analog processing portion 130 includes an input 132 coupled to theoutput 114 of the digital processing portion 110, to receive theprocessed and offset digital audio stream. The analog processing portion130 may optionally include digital circuitry to additionally process thestream in the digital domain. The analog processing portion 130 alsoincludes circuitry to convert the offset digital audio stream to theanalog domain, then process it still further in the analog domain, andfinally output an analog audio signal through an output port 134.Digital processing in the analog processing portion 130 may include, forexample, upsampling with a zero order hold (ZOH) circuit, and passingthe stream through a sigma delta modulator (also known as sigma deltanoise shaper). Conversion to the analog domain may be done by applying adata weighted averaging (DWA) algorithm, and driving a digital-to-analogconverter (DAC) with the stream. One or more analog components performthe analog processing. An analog component is any electronic componentthat receives, processes and/or outputs analog information signals, asopposed to digital values. Processing in the analog domain may includepassing the stream (which is now represented by one or more analogsignals) through a low pass filter, such as a switched capacitor biquadfilter.

The output port 134 may be coupled to a multiplexer (not shown), forswitching between or among different output amplifiers that aregenerally known in the art. The different output amplifiers may include,for example, a line power management integrate circuit (PMIC) poweramplifier, a line output power amplifier, a headphone (HPH) poweramplifier, and an earphone power amplifier.

The DC output offset introduced into the audio signal path by the analogcomponents of the analog processing portion 130 can be generally brokeninto two constituent parts: a systematic DC offset and a random DCoffset. If the total DC offsets of a large number of analog processingportions made with the same process were to be analyzed statistically,they would be expected to form a normal distribution, as is generallythe case with similar error statistics. However, a normal distributionof DC offsets in the analog processing portions made by the same processis not a requirement of the devices, apparatuses, methods and articlesof manufacture disclosed herein. The systematic DC offset of aparticular analog processing portion, for example, the portion 130, canthen be thought of as the mean of the distribution, and the random DCoffset can be thought of as the deviation from the statistical mean. Theprocess used to make to analog processing portions can be characterizedand the mean estimated from a production sample. Advantageously, thedigital offset input into the summer 120 can be set so as to negate thesystematic DC offset of the analog processing portion 130. In this way,the actual offset at the output of the analog processing portion 130 canbe reduced or eliminated.

As has already been mentioned, the audio path apparatus 100 may bereplicated for a second and subsequent sound channels, for example,right and left stereo channels. Moreover, the digital processing portion110 may itself be configurable, with two or even more selectableinternal paths, and with multiple summers for effecting digital offsetinto the digital audio sample stream. These concepts are illustrated inFIG. 2.

FIG. 2 a conceptual block diagram illustrating certain components of anexemplary audio device 200 configured to reduce or eliminate outputoffset from an audio signal path. The audio signal path can be a receivepath 201 of a coder/decoder (codec), which may be included in a digitalaudio device, such as a cellular access terminal (e.g., a cellularphone) with audio playback capability, e.g., MP3 playback. In thisembodiment, the receive path 201 includes two parallel stereo channels,right and left. In the left channel, the apparatus includes a leftdigital processing portion 210 and a left analog processing portion 230.In the right channel, the apparatus includes a right digital processingportion 260 and a right analog processing portion 280.

In the exemplary audio device 200, the received left and right audiosignals are preferably a stream of PCM signals. In each channel the PCMsignals are amplified in the digital domain by left and rightprogrammable gain amplifiers (PGAs) 240, 250, respectively. The Rx gaincan be a 16-bit value.

There are effectively two signal paths in each of the digital processingportions 210 and 260. Each digital processing portion 210, 260 includesa hardware path 273 and 275, respectively, and a firmware path 277 and279, respectively. The digital components of the hardware paths 273, 275are implemented using hardware components, whereas the components of thefirmware paths 277, 279 are implemented in software and/or firmware.Thus, the components of the firmware paths 277, 279 are programmable andchangeable through software programming techniques.

The hardware and firmware paths are selectable; in other words, thedigitized audio signal can be directed either through the firmware paths277 and 279, or alternatively, through the hardware paths 273 and 275. Aselector, such as switches 246, 248, 256, 258, allows selection of therespective hardware and firmware paths 273, 275, 277, 279. The switches246, 256 can be de-multiplexers and the switches 248, 258 can bemultiplexers, all under control of a state machine or processorconfigured to control operation of the audio device 200. Alternatively,the inputs and the outputs of the hardware and firmware paths 273, 275,277, 279 can be memory mapped registers or memory locations that arereadable/writable by the processor/state machine to effect the functionsof the switches 246, 248, 256, 258. An advantage of having both hardwareand firmware paths is that the digital processing portions 210, 260 arereadily configurable and updateable (at least the firmware paths 277,279) to handle different audio codec algorithms and schemes.

Focusing on the left channel digital processing portion 210, a firstleft summer 220 is at or near the end of the firmware path 277, whichalso includes truncators 216, 218 and a low pass (LP) finite impulseresponse (FIR) filter 217. A second left summer is located at or nearthe beginning of the hardware path 273. The hardware path 273 of thedigital processing portion 210 includes truncators 226 and 228, and anLP FIR filter 227. In a particular embodiment, the LP FIR 217 and the LPFIR 227 may each have 225 taps. The truncators 216, 218, 226, 228 eachtruncate a digital sample to a given number of significant bits,discarding the least significant bits. In the examples shown, truncators216 and 226 discard the 16 least significant bits of the samples, andtruncators 218, 228 discard the 13 least significant bits of thesamples.

The first and second left summers 220, 225 each add a digital offset tothe audio samples. The digital offset is stored in a memory 242, and canbe a PCM value representing the DC output offset introduced into theaudio signal by the analog processing portion 230. The digital offsetfor each path 273, 277 may have the same value or different values. Asdescribed above in connection with the apparatus of FIG. 1, the digitaloffset compensates for output offset caused by the analog components ofthe respective downstream analog processing portion 230.

The output of the left channel digital processing portion 210 isconnected to an input 232 of the left channel analog processing portion230. The analog processing portion 230 includes a zero order holdcircuit 235, a sigma delta modulator 236, a data weighted averagingalgorithm in combination with a digital-to-analog converter (DAC) 237,and a switched capacitor biquad low pass filter 238. Note that the ZOH235 and the sigma delta modulator 236 are implemented in hardware. Thenon-ideal operational characteristics of components in the analogprocessing portion 230, particularly analog components 237 and 238,introduce a DC output offset into the audio signal. The output offset isdetectable at the output of the analog processing portion 230.

As can be seen from FIG. 2, arrangements and functionality of the rightchannel digital processing portion 260 and the right channel analogprocessing portion 280 are identical or analogous to the arrangementsand functions of the left channel digital processing portion 210 and theleft channel analog processing portion 230, respectively. The rightchannel digital processing portion 260 includes switches 256, 258,firmware path 279, and hardware path 275. The firmware path 279 includestruncators 290, 292, an LP FIR filter 291, and summer 293; the hardwarepath 277 includes truncators 295, 297, and an LP FIR filter 296, withsummer 294 located at the beginning of the hardware path 275.

The summers 293, 294 each add a digital offset to the audio samples. Thedigital offset is stored in a memory 252, and can be a PCM valuerepresenting the DC output offset introduced into the right channelaudio signal by the right channel analog processing portion 280. Thedigital offset for each path 275, 279 may have the same value ordifferent values. As described above in connection with the apparatus ofFIG. 1, the digital offset compensates for output offset caused by theanalog components of the respective downstream analog processing portion280.

The right channel analog processing portion 280 includes a zero orderhold circuit 282, a sigma delta modulator 284, a data weighted averagingalgorithm in combination with a digital-to-analog converter (DAC) 286,and a switched capacitor biquad low pass filter 288. These right channelcomponents function in the same manner described above for thecorresponding components of the left channel analog processing portion230.

The analog audio signal outputs of the left and right channel analogprocessing portions 230, 280, are provided to de-multiplexers (DEMUXs)244 and 254, respectively. The de-multiplexers 244, 254 selectivelyprovide the left and right analog audio signals, respectively, todifferent audio power amplifier that include, for example, left andright line PMIC amplifiers 268, 274, line output power amplifiers 266,272, and headphone (HPH) power amplifiers 264, 270. The left channelde-multiplexer 244 can also provide the audio signal to an earphonepower amplifier 262.

Advantageously, each of the summers 220, 225, 293, 294 is preferablyimplemented in firmware and is placed at the interface between thehardware and firmware implemented components of the receive path 201. Inthis way, the hardware changes to preexisting components in the audiodevice 200 can be reduced when implementing the output offset reductiontechnique described herein.

The digital offsets may be the same or differ in value between left andright channels.

FIG. 3 is a process diagram illustrating an exemplary method 300 forprocessing an audio signal to suppress undesirable audible artifactscaused by an output offset. In step 310, a digitized audio stream isreceived in a digital processing portion configured to process thedigitized audio stream, such as one of the elements 110, 210, or 260 inFIGS. 1 and 2. The digitized audio stream corresponds to an input analogaudio stream.

At step 315, the digital processing portion also receives (or reads) adigital offset to be applied to the digitized audio stream. The digitalprocessing portion may perform various processing functions on thedigitized audio stream, such as upsampling and/or filtering, at step320. The digital processing portion also adds the digital offset to thedigitized audio stream, so that the processed digitized audio stream atthe output of the digital processing portion corresponds to an offsetand processed analog audio stream. This is done at step 325. The offsetand processed audio stream at the output of the digital processingportion is a processed and offset version of the input analog audiostream. The offset and processed audio stream is then transmitted to ananalog processing portion, at step 330. The offset and processed audiostream may be further processed in the digital domain by digitalcircuitry included within the analog processing portion, and thenconverted into an analog stream, at steps 335 and 340, respectively. Theanalog signal may be processed still further in the analog domain by oneor more analog components included within the analog processing portion,in step 345. For example, the analog signal may be filtered. The analogsignal may then be provided to an analog output for driving a headphone,earphone, speaker, and similar devices, at step 350.

Although the steps of the method in FIG. 3 have been described serially,some of these steps may be performed in parallel, in a pipelined manner,or otherwise. There is no particular requirement that the steps anddecisions be performed in the same order in which this description liststhem and FIG. 3 shows them, except where explicitly so indicated,otherwise made clear from the context, or inherently required, as wouldbe apparent to one of ordinary skill in the art.

The application of digital offset to cancel systematic output offset maybe phased out slowly over a period of time such that the phase out wouldnot be noticeable to the user. Similarly, the digital offset may beintroduced slowly before turning off the equipment, or before switchingthe mode of equipment, for example, when switching between telephone andplayer functions, or between different cellular systems. The slow phaseout and introduction of the digital offset may be done in some variants.For example, the digital offset may be introduced and phased outlinearly over a time period of 10 to 100 ms. But from a system point ofview, it may be preferable to keep the digital offset applied at all ormost times of operation, so that there is no need to wait beforechanging state or turning the audio device off. For example, a wait timeon the order of 100 ms between switching states may be both noticeableand annoying to the user. Therefore, in some variants, the digitaloffset is applied at all or substantially all times of sound operation.

In audio devices operating in ground-referenced capless mode, powerconsumption is reduced by the application of the digital offset, becauseof the reduced or eliminated DC leakage through speakers or headsets ofsuch devices. This is yet another reason for not phasing out digitaloffset. Each of the embodiments described herein may be specificallyimplemented to support a ground-referenced capless mode of operation.Ground-referenced capless audio output devices are well known in theart.

FIG. 4 a conceptual block diagram illustrating certain components ofanother exemplary audio device 400 configured to reduce or eliminateoutput offset from an audio signal path 401 a, 401 b. The audio device400 includes one or more feedback paths 403 for providing the outputoffset of the analog processing portion to the digital processingportion. The feedback path 403 allows the output offset to be measuredand the corresponding digital offset to be dynamically updated in anindividual audio device, rather than having a static digital offsetbased on a statistically determined estimation of the output offset. Thedigital offset can be periodically calculated and updated based on themeasured output offset. For example, on power up, at mode changes and/orat predetermined intervals, the feedback path 403 can be used to measurethe DC output offset at the analog portion output and update the digitaloffset. Thus, the feedback path 403 may allow for more accuratesuppression of the output offset.

The audio signal path 401 a, 401 b can be a receive path of a codec,which may be included in a digital audio device, such as a cellularaccess terminal (e.g., a cellular phone) with audio playback capability,e.g., MP3 playback. The receive path includes two parallel stereochannel paths, right 401 b and left 401 a. The received left and rightaudio signals are preferably streams of PCM signals. In the left channelpath, the audio device 400 includes a left channel processing portion402, which includes a left channel summer 406, a digital-to-analogconverter (DAC) 408, an analog low-pass filter (LFP) 410, a left HPHamplifier (Amp) 412, a left output offset register 414, and a leftaverage output offset buffer 416. In the right channel path, the audiodevice 400 includes a right channel processing portion 404, whichincludes a right channel summer 430, a DAC 432, an analog LFP 434, aright HPH amplifier (Amp) 436, a right output offset register 438, and aright average output offset buffer 440. The right and left channelspaths can include additional digital and/or analog components, such asthose discuss above in connection with FIGS. 1 and 2.

The left and right channel summers 406, 430 can each function and beimplemented in the same manner as described above for summers 220, 225,293, 294 shown in FIG. 2.

The example feedback path 403 shown in FIG. 4 includes a MUX 420, an amp422, an anti-aliasing filter 424, an analog-to-digital converter (ADC)426, and a de-multiplexer (DEMUX) 428. The feedback path 403 can be partof or share components with a transmit (Tx) path through a codec.

The state machine 418 controls operation of the feedback path 403,measurement and processing of the output offset, and determination ofthe digital offset. The state machine 418 can be implemented using anysuitable combination of hardware and/or software technology. Preferably,the state machine 418 is implemented in software/firmware executing on aprocessor.

As input, the feedback path 403 can selectively receive through the MUX420 either the left channel output offset on line 407 a, the rightchannel output offset on line 407 b, or a Tx analog audio input, such asinput from a microphone. The output of the feedback path 403 can beselectively provided to the left average output offset buffer 416, theaudio Tx path, or the right average output offset buffer 440, throughthe DEMUX 428. The MUX 420 and DEMUX 428 are controlled by the statemachine 418 through enable signals (Enable L/R).

Prior to enabling the right and left headphone amps 436, 412 (usingEnable_R_PA and Enable_L_PA signals, respectively), the state machine418: 1) sets the MUX 420 and DEMUX 428 to route the left channel analogLPF output on line 407 a through MUX 420, amp 422, anti-aliasing filter424, ADC 426, and DEMUX 428 to the left average output offset buffer416; 2) waits a predefined period of time to allow the left channeloutput offset to propagate through the feedback path 403 (e.g., 10milliseconds); 3) latches the measured output offset value at the outputof the ADC 426 into the left average output offset buffer 416; 4)averages the DC output offset value over a predetermined number ofoutput offset samples or period of time; and 5) sets the left outputoffset register 414 to the average output offset value finally computedand placed in the buffer 416.

The above steps 1-5 are repeated for the right channel using the rightchannel output offset measured on line 407 b, resulting in the rightdigital offset register 438 being loaded with a right channel averageoutput offset value. After loading both the left and right output offsetregisters 414, 438, the state machine 418 waits a predefined period oftime to allow offset audio signals on both left and right channels topropagate through to the outputs of the respective analog LPFs 410, 434.The state machine 418 then sets the MUX 420 and DEMUX 428 to enable theaudio input (e.g., microphone input) to flow through the feedback pathcomponents to the Tx path. The state machine 418 then enables the leftand right HPH amps 412, 436 to provide offset audio signals to therespective headphones 442, 444.

The digital offset can be adjusted by the state machine 418 tocompensate for alterations to the audio signal by analog components. Forexample, the digital offset can comprise a predetermined number of bits,where the number of bits is based on gain applied to the audio signal bythe analog components.

Although described above in connection with a two channel audio device,the technique of feeding back a measured output offset can be applied toa single audio channel device.

The apparatus 100, audio devices 200, 400 and method 300 described abovemay each be included in an access terminal, which also may be referredto as AT, subscriber station, user equipment, UE, mobile terminal, MT,or cellular communication device, may be mobile or stationary, and maycommunicate with one or more base transceiver stations. An accessterminal may be any of a number of types of devices, including but notlimited to personal computer (PC) card, external or internal modem,wireless telephone, and personal digital assistant (PDA) with wirelesscommunication capability. An access terminal transmits and receives datapackets to or from a radio network controller through one or more basetransceiver stations.

Some cellular access terminals can communicate with different kinds ofcommunication networks, for example, with both CDMA networks and GlobalSystem for Mobile communications (GSM) networks. Some access terminalsmay also provide additional audio-related functionality, such as audioplayback.

A CDMA system may be designed to support one or more CDMA standards,such as (1) the “TIA/EIA-95 Mobile Station-Base Station CompatibilityStandard for Dual-Mode Wideband Spread Spectrum Cellular System” (thisstandard with its enhanced revisions A and B may be referred to as the“IS-95 standard”), (2) the “TIA/EIA-98-C Recommended Minimum Standardfor Dual-Mode Wideband Spread Spectrum Cellular Mobile Station” (the“IS-98 standard”), (3) the standard sponsored by a consortium named “3rdGeneration Partnership Project” (3GPP) and embodied in a set ofdocuments known as the “W-CDMA standard,” (4) the standard sponsored bya consortium named “3rd Generation Partnership Project 2” (3GPP2) andembodied in a set of documents including “TR-45.5 Physical LayerStandard for cdma2000 Spread Spectrum Systems,” the “C.S0005-A UpperLayer (Layer 3) Signaling Standard for cdma2000 Spread SpectrumSystems,” and the “TIA/EIA/IS-856 cdma2000 High Rate Packet Data AirInterface Specification” (the “cdma2000 standard” collectively), (5) the1xEV-DO standard, and (6) certain other standards.

The various methods, apparatuses, components, functions, state machines,devices and circuitry described herein may be implemented in hardware,software, firmware or any suitable combination of the foregoing. Forexample, the methods, apparatuses, components, functions, statemachines, devices and circuitry described herein may be implemented, atleast in part, with one or more general purpose processors, digitalsignal processors (DSPs), application specific integrated circuits(ASICs), field programmable gate arrays (FPGAs), intellectual property(IP) cores or other programmable logic devices, discrete gates ortransistor logic, discrete hardware components, or any combinationthereof designed to perform the functions described herein. A generalpurpose processor may be a microprocessor, but in the alternative, theprocessor may be any conventional processor, controller,microcontroller, or state machine. A processor may also be implementedas a combination of computing devices, e.g., a combination of a DSP anda microprocessor, a plurality of microprocessors, one or moremicroprocessors in conjunction with a DSP core, or any other suchconfiguration.

The functions, state machines, components and methods described herein,if implemented in software, may be stored or transmitted as one or moreinstructions or code on a computer-readable medium. Computer-readablemedia includes both computer storage media and communication mediaincluding any medium that facilitates transfer of a computer programfrom one place to another. A storage media may be any available mediathat can be accessed by a computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code in the form of instructions or datastructures and that can be accessed by a computer processor. Also, anytransfer medium or connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above are also includedwithin the scope of computer-readable media.

The above description of the disclosed embodiments is provided to enableany person skilled in the art to make or use that which is defined bythe appended claims. The following claims are not intended to be limitedto the disclosed embodiments. Other embodiments and modifications willreadily occur to those of ordinary skill in the art in view of theseteachings. Therefore, the following claims are intended to cover allsuch embodiments and modifications when viewed in conjunction with theabove specification and accompanying drawings.

What is claimed is:
 1. An apparatus, comprising: a digital processingportion including: a first truncator; a filter configured to receiveoutput from the first truncator; a second truncator configured toreceive output from the filter; and a summer to add a digital offset toa digitized audio signal output from the second truncator, wherein thedigital offset is based on an output offset introduced into an analogaudio signal by one or more analog components configured to produce theanalog audio signal in response to the digitized audio signal.
 2. Theapparatus of claim 1, wherein the digital offset corresponds to thenegative of the output offset.
 3. The apparatus of claim 1, wherein theoutput offset is a predetermined systematic DC offset.
 4. The apparatusof claim 1, further comprising: a feedback path configured to providethe output offset to the digital processing portion.
 5. The apparatus ofclaim 4, further comprising: an analog-to-digital converter (ADC)included in the feedback path, configured to convert the output offsetto a digital value.
 6. The apparatus of claim 1, further comprising: anoffset measuring portion configured to determine the output offset uponinitialization of the apparatus.
 7. The apparatus of claim 1, whereinthe digital processing portion comprises: a firmware path; a hardwarepath; and a selector configured to select either the firmware path orthe hardware path for processing the digitized audio signal.
 8. Theapparatus of claim 1, further comprising: an audio amplifier configuredto receive the audio signal from at least one of the analog components.9. The apparatus of claim 1, further comprising: a multiplexerconfigured to receive the audio signal output from at least one of theanalog components; and a plurality of audio amplifiers configured toselectively receive the audio signal from the multiplexer.
 10. Theapparatus of claim 1, further comprising a programmable gain amplifier(PGA) configured to amplify the digitized audio signal.
 11. Anapparatus, comprising: means for storing a digital offset, wherein thedigital offset is based on an output offset introduced into an audiosignal by one or more analog components; and means for adding thedigital offset to a digitized audio signal prior to the digitized audiosignal being received by the analog components to produce an offsetdigitized audio signal; means for converting the offset digitized audiosignal into an analog audio signal using a multi-bit digital-to-analogconverter (DAC).
 12. The apparatus of claim 11, wherein the digitaloffset corresponds to the negative of the output offset.
 13. Theapparatus of claim 11, further comprising: means for measuring theoutput offset upon initialization of the analog components; and meansfor converting the output offset to the digital offset.
 14. Theapparatus of claim 13, further comprising: feedback means for providingthe output offset to the storing means.
 15. The apparatus of claim 11,further comprising: means for selecting either a firmware path or ahardware path within a digital processing portion for processing thedigitized audio signal.
 16. A device for suppressing transient audibleartifacts, comprising: digital circuitry configured to add a digitaloffset to a digitized audio signal to produce an offset digitized audiosignal; a digital-to-analog converter (DAC) receiving the offsetdigitized audio signal; and analog circuitry configured to receiveoutput from the DAC and to process the output to produce an analog audiosignal; wherein the analog circuitry includes an amplifier inground-referenced mode for amplifying the analog audio signal; whereinthe digital offset is based on an output offset introduced into theanalog audio signal by the analog circuitry.
 17. The device of claim 16,wherein the digital offset corresponds to the negative of the outputoffset.
 18. The device of claim 16, wherein the digital circuitrycomprises: a firmware path; a hardware path; and a selector configuredto select either the firmware path or the hardware path for processingthe digitized audio signal.
 19. The device of claim 16, furthercomprising: a feedback path from the analog circuitry to the digitalcircuitry, configured to provide the output offset to the digitalcircuitry.
 20. The device of claim 19, further comprising: ananalog-to-digital converter (ADC) included in the feedback path,configured to convert the output offset to a digital value.
 21. Thedevice of claim 16, further comprising: a multiplexer configured toreceive the audio signal output from the analog circuitry; and aplurality of audio amplifiers configured to selectively receive theaudio signal from the multiplexer.
 22. An apparatus, comprising: a firstdigital processing portion configured to add a first digital offset to afirst digitized audio signal corresponding to a left stereo channel; afirst analog processing portion configured to receive the firstdigitized audio signal from the first digital processing portion and toprocess the first digitized audio signal to produce an analog audiosignal, the first analog portion including a first audio amplifier inground-referenced mode for amplifying the analog audio signal, a firstfilter having a first output and means for providing the first output asthe first digital offset; a second digital processing portion configuredto add a second digital offset to a second digitized audio signalcorresponding to a right stereo channel; and a second analog processingportion configured to receive the second digitized audio signal from thesecond digital processing portion and to process the second digitizedaudio signal to produce an analog audio signal, the second analogportion including a second audio amplifier in ground-referenced mode foramplifying the analog audio signal, a second filter having a secondoutput and means for providing the second output as the second digitaloffset; wherein the first digital offset is based on a first outputoffset introduced into the first analog audio signal by the first analogprocessing portion; wherein the second digital offset is based on asecond output offset introduced into the second analog audio signal bythe second analog processing portion.
 23. The apparatus of claim 22,wherein the first digital offset corresponds to the negative of thefirst output offset; and wherein the second digital offset correspondsto the negative of the second output offset.
 24. The apparatus of claim11, further comprising: means for averaging a plurality of measuredoffset values to obtain the digital offset.
 25. The apparatus of claim11, further comprising: means for amplifying the analog audio signal inground-reference mode.
 26. The device of claim 16, wherein the analogcircuitry includes an amplifier in ground-reference mode for amplifyingthe analog audio signal.
 27. The device of claim 16, wherein the analogcircuitry includes a low-pass filter (LPF) having an output measured toobtain the digital offset.
 28. An apparatus, comprising: a digitalprocessing portion including: a summer to add a digital offset to adigitized audio signal; a first truncator receiving output from thesummer; a filter configured to receive output from the first truncator;and a second truncator configured to receive output from the filter;wherein the digital offset is based on an output offset introduced intoan analog audio signal by one or more analog components configured toproduce the analog audio signal in response to the digitized audiosignal.
 29. The apparatus of claim 28, further comprising: a feedbackpath configured to provide the output offset to the digital processingportion.
 30. The apparatus of claim 29, further comprising: ananalog-to-digital converter (ADC) included in the feedback path,configured to convert the output offset to a digital value.
 31. Theapparatus of claim 28, further comprising: an offset measuring portionconfigured to determine the output offset upon initialization of theapparatus.
 32. The apparatus of claim 28, wherein the digital processingportion comprises: a firmware path; a hardware path; and a selectorconfigured to select either the firmware path or the hardware path forprocessing the digitized audio signal.
 33. An apparatus, comprising: adigital processing portion configured to add a digital offset to adigitized audio signal, wherein the digital offset is based on an outputoffset introduced into an analog audio signal by one or more analogcomponents configured to produce the analog audio signal in response tothe digitized audio signal; and an analog processing portion includingthe analog components, comprising: a zero-order hold circuit; asigma-delta modulator configured to receive output from the zero-orderhold circuit; a digital-to-analog converter (DAC) configured to receiveoutput from the sigma-delta modulator; and a filter configured toreceive output from the DAC.
 34. The apparatus of claim 33, furthercomprising: a feedback path configured to provide the output offset tothe digital processing portion.
 35. The apparatus of claim 34, furthercomprising: an analog-to-digital converter (ADC) included in thefeedback path, configured to convert the output offset to a digitalvalue.
 36. The apparatus of claim 33, further comprising: an offsetmeasuring portion configured to determine the output offset uponinitialization of the apparatus.
 37. The apparatus of claim 33, whereinthe digital processing portion comprises: a firmware path; a hardwarepath; and a selector configured to select either the firmware path orthe hardware path for processing the digitized audio signal.